Plating by an electroless plating method utilizes a reduction reaction induced by contact with a material surface so that plating can be performed with a uniform thickness, even on receding zones. In particular, because electroless Ni plating typically excels in corrosion resistance and wear resistance, the method has been used for a long time for surface treatment of raw material components, and the history thereof is long. Presently, this method is widely used for the primary treatment of solder joints of printed wiring boards or primary treatment of compact disks (CD) and hard disk drives (HDD).
Because lead compounds are introduced as stabilizers in electroless nickel plating liquids that have been generally used for the primary treatment of solders, the nickel film obtained also contains lead.
However, recent establishment of the RoHS Directive in the EU (European Union) placed further limitations on hazardous substances such as lead and chromium in electronic components (presently, 0.1% or less for lead), and these restrictions are expected to be even more stringent in the future. The conventional types of solders are generally eutectics of tin and lead, but in recent years lead-free two-component or three-component solders such as tin-silver-zinc and tin-silver-bismuth were put to practical use. Because the restrictions of the RoHS Directive are applied not only to a solder, but also to the entire electronic devices, these restrictions are also valid for a nickel film obtained by an electroless process that has generally been used widely for primary treatment for soldering. For example, the restrictions of the RoHS Directive have also to be taken into consideration in the electroless nickel plating method described in Patent Document 1 that improved the corrosion resistance of a nickel coating and prevented a decrease in solder wettability.
When a nickel underbarrier metal (UBM) for solder bumps or Ni metal bumps (protruding sections) is formed by electroless nickel plating on a silicon wafer composed of multiple IC chips, new problems arise: the electric potential difference inside the integrated circuit (for example, if a p-type semiconductor is formed by further doping boron into a n-type semiconductor (Si doped with microquantity of phosphorus), a n/p diffusion layer appears at the junction surface, then, an electric potential difference of about 0.4 V appears between P/N electrodes by exposing the IC to light of 100 Lux); due to fine electrode pads, the height of the nickel metal deposited on the electrode pad is uneven and, in the worst case scenario, absolutely no nickel metal is deposited. Consequently, since a large number of problems have not yet been solved, electroless nickel liquids are considered to be difficult to use as silicon wafer bumps or UBN.
For this reason, at present, Au bumps with a height of about 15 μm are fabricated by a gold (Au) electroplating method or a barrier metal with a height of about 5 μm is fabricated by using a combination of sputtering and electroplating for UBM. However, the Au plating method involves a complex process and a high cost. Because the sputtering and electroplating methods require etching of the diffusion preventing layer and seed layer to which electricity is supplied, the process is complex and productivity is low.    Patent Document 1: Japanese Patent No. 3479639 Publication